The present invention relates generally to tuning of electronic filters and more particularly to a device and method for mechanically tuning a filter over a wide range of frequencies using one or more tuning pin.
Electronic filters are extensively used in a variety of applications, including wireless communication. Modern wireless communication systems impose stringent requirements on filter performance. Escalating demand for communication channels dictates more efficient use of allocated spectral slots. Increasingly, applications require very narrow-band bandpass filters (as narrow as 0.05% bandwidth) with precise center frequencies and high signal throughput within the bandwidth, as well as bandstop filters with precise center frequencies and high rejection. In either case, the filter response curve must have sharp skirts so that a maximum amount of the available bandwidth may be utilized without unacceptable levels of cross-talk. The filters must also have high quality factors, or Q-factors, (for example, 5,000 or higher) to minimize signal loss.
Various tuning means have been used to achieve precise filter center frequencies. For example, U.S. Pat. No. 5,968,876, which is assigned to Conductus, Inc., discloses tuning the resonant frequency of a resonator using a spring-loaded tuning pin having a superconducting, dielectric or magnetic tuning tip. The resonant frequency is adjusted by adjusting the distance between the tuning tip and the resonator. Such tuning mechanisms have typically been used to maintain center frequencies of filters but not substantially changing the center frequencies because the range of center frequencies that is achievable by a filter while maintaining other performance parameters is typically very small. For example, for a tuning device of the type disclosed in the U.S. Pat. No. 5,968,876, the tuning tip typically is not designed to travel closer to the resonator than about 0.5 mm. The tuning range is typically a few megahertz for center frequencies of the order of 850 MHz to 1.9 GHz.
The invention disclosed herein makes possible wide range tuning of filters to suit a broad range of applications.
Generally, the invention provides a tuning assembly and method for mechanically tuning planar resonators and filters over a wide range of frequencies with precision while maintaining the performance of the resonators and filters.
According to one aspect of the invention, the tuning assembly includes a tuning tip and an actuator configured and adapted to move the tuning tip through a sufficient range of distances from a resonator or a portion thereof to vary the resonant frequency of the resonator by at least about 10% of the untuned frequency in at least one increment of 0.01% or smaller.
According to another aspect of the invention, an actuator and a position sensing device are employed in a closed loop feedback system to control the position of the tuning tips.
According to another aspect of the invention, a method of tuning a filter includes moving a tuning tip through a sufficient range of distances from a resonator to vary the resonant frequency of the resonator by at least about 10% in at least one increment of 0.01% or smaller.